The present invention relates to a method of maintaining or widening the long-term operating temperature range of a heat transfer medium and/or heat storage medium as defined in the claims, a corresponding process system as defined in the claims, the use of an additive for maintaining or widening the long-term operating temperature range of a heat transfer medium and/or heat storage medium as defined in the claims and also a method of generating electric energy in a solar thermal power station as defined in the claims.
Heat transfer media or heat storage media based on inorganic solids, in particular salts, are known both in chemical technology and in power station technology. They are generally used at high temperatures, for example above 100° C., thus above the boiling point of water at atmospheric pressure.
For example, salt bath reactors are used at temperatures of from about 200 to 500° C. in chemical plants for the industrial production of various chemicals.
Heat transfer media are media which are heated by an energy source, for example the sun in solar thermal power stations, and transport the heat comprised therein over a particular distance. They can then transfer this heat to another medium, for example water or a gas, preferably via heat exchangers, with this other medium then being able, for example, to drive a turbine. Heat transfer media can also be used in chemical process technology to heat or cool reactors (for example salt bath reactors) to the desired temperature.
However, heat transfer media can also transfer the heat comprised therein to another medium (for example a salt melt) present in a reservoir and thus pass on the heat for storage. However, heat transfer media can themselves also be introduced into a reservoir and remain there. They are then themselves both heat transfer media and heat storage media.
Heat stores comprise heat storage media, usually materials compositions, for example the mixtures according to the invention, which can store heat for a particular time. Heat stores for fluid, preferably liquid, heat storage media are usually formed by a solid vessel which is preferably insulated against loss of heat.
A still relatively recent field of application for heat transfer media or heat storage media are solar thermal power stations for generating electric energy.
An example of a solar thermal power station is shown schematically in FIG. 1.
In FIG. 1, the numerals have the following meanings:
1 Incoming solar radiation
2 Receiver
3 Stream of a heated heat transfer medium
4 Stream of a cold heat transfer medium
5a Hot part of a heat storage system
5b Cold part of a heat storage system
6 Stream of a hot heat transfer medium from the heat storage system
7 Stream of a cooled heat transfer medium into the heat storage system
8 Heat exchanger (heat transfer medium/steam)
9 Steam stream
10 Condensate stream
11 Turbine with generator and cooling system
12 Current of electric energy
13 Waste heat
In a solar thermal power station, focused solar radiation (1) heats a heat transfer medium, usually in a receiver system (2) which usually comprises a combination of tubular “receivers”. The heat transfer medium generally flows, usually driven by pumps, firstly into a heat storage system (5a), flows from there via line (6) on to a heat exchanger (8) where it gives off its heat to water and thus generates steam (9) which drives a turbine (11) which finally, as in a conventional electric power station, drives a generator for generating electric energy. In the generation of electric energy (12), the steam loses heat (13) and then generally flows back as condensate (10) into the heat exchanger (8). The cooled heat transfer medium generally flows from the heat exchanger (8) back via the cold region (5b) of a heat storage system to the receiver system (2) in which it is reheated by solar radiation and a circuit is formed.
The storage system can comprise a hot tank (5a) and a cold tank (5b), for example as two separate vessels.
An alternative construction of a suitable storage system is, for example, a layer store having a hot region (5a) and a cold region (5b), for example in a vessel.
Further details regarding solar thermal power stations are described, for example, in Bild der Wissenschaft, 3, 2009, pages 82 to 99, and also below.
Three types of solar thermal power stations are particularly important at present:
the parabolic trough power station, the Fresnel power station and the tower power station.
In the parabolic trough power station, the solar radiation is focused via parabolic mirror troughs on the focal line of the mirrors. There, there is a tube (usually referred to as “receiver”) filled with a heat transfer medium. The heat transfer medium is heated by the solar radiation and flows to the heat exchanger where, as described above, it transfers its heat for steam generation. The parabolic trough-tube system can reach a length of over 100 kilometers in present-day solar thermal power stations.
In the Fresnel power station, the solar radiation is focused onto a focal line by generally flat mirrors. At the focal line there is a tube (usually referred to as “receiver”) through which a heat transfer medium flows. In contrast to the parabolic trough power station, the mirror and the tube are not moved together to follow the position of the sun, but instead the setting of the mirrors is offered relative to the fixed tube. The setting of the mirrors follows the position of the sun so that the fixed tube is always located on the focal line of the mirrors. In Fresnel power stations, too, molten salt can be used as heat transfer medium. Salt Fresnel power stations are at present largely still in development. Steam generation or the generation of electric energy in the salt Fresnel power station occurs in a manner analogous to the parabolic trough power station.
In the case of the solar thermal tower power station (hereinafter also referred to as tower power station), a tower is encircled by mirrors, in the technical field also referred to as “heliostats”, which radiate the solar radiation in a focused manner onto a central receiver in the upper part of the tower. In the receiver, which is usually made up of bundles of tubes, a heat transfer medium is heated and this produces, via heat exchangers, steam for generating electric energy in a manner analogous to the parabolic trough power station or Fresnel power station.
Heat transfer media or heat storage media based on inorganic salts have been known for a long time. They are usually used at high temperatures at which water is gaseous, i.e. usually at 100° C. and more.
Known heat transfer media or heat storage media which can be used at relatively high temperatures are compositions comprising alkali metal nitrates and/or alkaline earth metal nitrates, optionally in admixture with alkali metal nitrites and/or alkaline earth metal nitrites.
Examples are the products of Coastal Chemical Company LLC Hitec® Solar Salt (potassium nitrate:sodium nitrate 40% by weight:60% by weight), Hitec® (eutectic mixture of potassium nitrate, sodium nitrate and sodium nitrite).
The nitrate salt mixtures or the mixtures of nitrate and nitrite salts can be used at relatively high long-term operating temperatures without decomposing.
In principle, such mixtures which have a relatively low melting point or relatively high decomposition temperatures can be produced by the combination of nitrate salts, usually those of the alkali metals lithium, sodium, potassium, optionally together with nitrite salts, usually those of the alkali metals lithium, sodium, potassium or of the alkaline earth metal calcium.
In the following, the term alkali metal refers to lithium, sodium, potassium, rubidium, cesium, preferably lithium, sodium, potassium, particularly preferably sodium, potassium, unless expressly indicated otherwise.
In the following, the term alkaline earth metal refers to beryllium, magnesium, calcium, strontium, barium, preferably calcium, strontium, barium, particularly preferably calcium and barium, unless expressly indicated otherwise.
It is still an objective to develop a heat transfer medium or heat storage medium which becomes solid (solidifies) at a relatively low temperature, thus has a low melting point, but has a high maximum long-term operating temperature (analogous to a high decomposition temperature).
For the present purposes, the maximum long-term operating temperature is the highest operating temperature for the heat transfer medium or heat storage medium at which the properties of the medium, for example viscosity, melting point, corrosion behavior, do not change significantly compared to the initial value over a long period of time, in general from 10 to 30 years.
Preference is given to using mixtures of sodium nitrate or potassium nitrate at relatively high temperatures. A routine long-term operating temperature range is from 290 to 565° C. Such mixtures have a relatively high melting point.
Mixtures of alkali metal nitrate and alkali metal nitrite usually have a lower melting point than the abovementioned nitrate mixtures, but also a lower decomposition temperature. Mixtures of alkali metal nitrate and alkali metal nitrite are usually employed in the temperature range from 150° C. to 450° C.
However, it is desirable, in particular for use in power stations for generating electric energy, e.g. solar thermal power stations, to increase the temperature of the heat transfer medium to far above 400° C., for example to far above 500° C., on arrival in the heat exchanger of the steam generator (known as steam inlet temperature) since the efficiency of the steam turbine is then increased.
It is thus desirable to increase the thermal stability of heat transfer media in long-term operation to, for example, more than about 565° C.
The chemical and physical properties of nitrate salt mixtures and nitrate/nitrite salt mixtures and thus, for example, their long-term operating temperature range in solar thermal power stations can change in an adverse manner in a number of ways, for example, when the abovementioned mixtures are subjected, in particular over a prolonged period of time, to comparatively high temperatures, for example above 565° C. in the case of nitrate salt mixtures and above 450° C. in the case of nitrate/nitrite salt mixtures. They generally then decompose into various degradation products.
This generally results in a decrease in the maximum long-term operating temperatures to below an economically and/or technically acceptable value and/or an increase in the melting point to above an economically and/or technically acceptable value. Furthermore, the decomposition of the mixtures mentioned usually also results in an increase in their corrosiveness.
Furthermore, the chemical and physical properties of nitrate salt mixtures and nitrate/nitrite salt mixtures and thus, for example, their long-term operating temperature range in solar thermal power stations can change in an adverse manner as a result of uptake of traces or even relatively large amounts of water or carbon dioxide, for example due to a leak in the heat transfer medium/steam heat exchanger or as a result of open operation in which the heat transfer media or heat storage media are in contact with the atmospheric moisture of the outside air.
The properties of the nitrate salt mixtures or nitrate/nitrite salt mixtures can in this way deteriorate to such an extent that they become unsuitable as heat transfer medium or heat storage medium and generally have to be replaced by fresh mixtures, which in the case of the huge amounts comprised in, for example, the piping and storage system of a solar thermal power station having multihour thermal stores is technically and economically disadvantageous or virtually impossible.
It was an object of the present invention to discover a method which avoids or reverses the deterioration of a heat transfer medium or heat storage mediums based on a nitrate salt mixture or a nitrate/nitrite salt mixture or widens the long-term operating temperature range of such mixtures.
A further object of the present invention was to discover a method which makes a nitrite salt-comprising heat transfer medium or heat storage medium suitable for higher long-term operating temperatures.